REPRODUCTIVE CELLS. 121 



After a time the wall of the parent-cell is ruptured or becomes 

 absorbed, and the young microspores come to lie free in the 

 pollen-chambers or sacs (microsporangia), which are lined by the 

 remains of the tapetum. The immature microspore presents the 

 following features : — 



i. A thin eell-Wall externally. 



ii. Internally, granular cytoplasm, in which are a large nucleus and 

 food-granules (starch, &c. ). 



The wall soon becomes modified so as to consist of two 

 distinct layers (see Fig. 91, e) — viz., an outer one, the extine, 

 which is thick, and often beautifully marked by reticulations, 

 projections of various shapes, or thin-wall areas ; and an inner 

 one, the intine, which encloses the cytoplasm. From the mark- 

 ings on the extine it is often possible to distinguish the genus or 

 species from which the pollen was derived. 



Maturation of the microspore consists in the formation of 

 •certain cells by the division of the cytoplasm and nucleus of the 

 main cell. At first a small cell is occasionally cut off, which is 

 known as the prothallial cell, the significance of which will be 

 pointed out when the Heterosporous Ferns are considered. Next, 

 a cell is cut off from the remaining larger cell ; this is the 

 so-called " generative " cell, and is the effective male cell in fer- 

 tilisation. Thus, in the mature microspore there are present 

 three cells enclosed within the intine (see Fig. 91, e), viz.: — 



u. The prothallial cell ; this cell is generally absent. It may, however, 

 e seen in the microspore of Sparganium. 



/3. The generative cell, 



y. The vegetative cell, this being the large cell left after the forma- 

 ation of a and /3. It is this cell which forms the pollen-tube during 

 fertilisation. 



All these maturation changes in the microspore may take place 

 whilst it is resting on the stigma of the ovary. The further 

 changes which take place — viz , formation of the pollen-tube and 

 division of the generative nucleus — are best considered under 

 fertilisation. 



b. The Macrospore : its Origin and Maturation. — The macro- 

 spore or embryo-sac is contained at the apex of the nucellus of 

 the ovule (macrosporangium) in Angiosperms, and has the 

 following origin : — The terminal hypodermal cell of the axial row 

 of cells in the young nucellus is the so-called archesporial cell or 



